1. Field of the Invention
The present invention relates generally to polycyclic olefin polymers and particularly to polycyclic olefin polymers prepared with non-olefinic chain transfer agents and uses of such polymers.
2. Description of Related Art
To meet the requirement of the semiconductor industry for smaller device sizes to produce faster and more complex devices, improvements in the resolution capabilities of current photolithographic process must be made. One area being explored for such improvements is the use of short wavelength exposure sources, for example sources that provide wavelengths of 193 nanometers (nm) and 157 nm, since it is known that imaging resolution increases as the wavelength of the imaging radiation decreases. However, current photoresist compositions which are employed for longer wavelength radiation sources, 248 nm and above, are generally not usable for exposure at 193 nm and below, as they are too absorptive at such short wavelengths. Further, the optical density (OD) of such current resist compositions is too high at wavelengths of 193 nm and below to be viable for exposure at such wavelengths. Therefore, to take advantage of the resolution benefit such short wavelength radiation affords, new photoresist compositions that have sufficiently low ODs at 193 nm and below must be made available.
Poly(cyclic) olefin polymers, such as those including norbornene-type structures, have shown promise for use in photoresist compositions suitable for exposure at wavelengths such as 197 nm and 153 nm. However, such materials must also possess other characteristics in order to make them truly viable. For example, a positive acting (positive tone) photoresist should have a high dissolution rate after exposure and post-exposure thermal treatment, as such high dissolution rates are both a factor in development of the image; that is to say removing the exposed regions of photoresist and providing for high wafer throughput in an integrated circuit manufacturing environment. It is generally known that low molecular weight polymers, such as those used for photoresist compositions, tend to exhibit higher dissolution rates than their higher molecular weight analogs. Unfortunately, it is also known that such low molecular weight materials generally have a higher OD than their higher molecular weight analogs. (See, Barclay et. al. Macromolecules 1998, 31, 1024 for a discussion of these issues for poly(4-hydroxystyrene), the preferred material for 248 nm photoresists.) As a result, it is often necessary for a person formulating such a polymer to target a higher molecular weight than desired for an optimal dissolution rate so that an acceptable OD can be obtained. It follows then that this compromise between optimal dissolution rate and optimal OD results in a photoresist composition that is not optimized for either characteristic.
One method of overcoming the results of this compromise is through the use of optically transparent dissolution rate modifiers (DRMs), a material that can be added to the photoresist composition to enhance the dissolution rate in the exposed areas of the resist. While such DRMs are useful, providing such materials is an extra step that increases both the complexity of the resist composition and its cost. Hence, it would be advantageous to be able to provide a polymer for a photoresist composition that is optimized for both a low OD at the target wavelength and a low molecular weight, thus making the use of a DRM in such compositions an optional material to further enhance the composition's performance.
One method for providing norbornene-type polymers with controllably low molecular weights was disclosed in U.S. Pat. No. 5,468,819, incorporated herein by reference. This method employs olefinic chain transfer agents (CTAs) such as ethylene, 1-hexene, 1-decene, and the like, with both nickel and palladium catalysts. While this method has been successful for providing controllably low molecular weight polymers, it was observed that the OD of the resulting polymers tended to increase as the molecular weight of the polymers decreased.
FIG. 1 shows a plot of OD (at 193 nm) versus molecular weight (Mw) for homopolymers of α,α-bis(trifluoromethyl)bicyclo[2.2.1]hept-5-ene-2-ethanol (HFANB). The plot shows the trend of OD increasing as the molecular weight of the polymers decreases. Such an increase is believed to be due to the incorporation of olefinic terminal groups into the resulting polymer. It was subsequently found that when the olefinically terminated cyclic olefin polymers were treated (after polymerization) with, for example, peracetic acid, the OD was lowered. It is believed that the peracetic acid lowers the OD by converting the highly absorbing olefinic terminal groups into less absorbing epoxides. Referring to FIG. 2, the result of a peracetic acid post-treatment on homopolymers of HFANB is shown by comparing the data points of FIG. 1 with additional data points reflective of OD after peracid post-treatment. Treatment with peracetic acid results in a reduced optical density. The same trend has been observed for polymers of HFANB and the t-butylester of 5-norbornene carboxylic acid (t-BuEsNB).
While post-treatment of olefinically terminated poly(cyclic) olefins with, for example, peracetic acid successfully creates polymers with a low OD at 193 nm, it requires an additional synthetic step after polymerization, an epoxidation. It would be desirable then to find an alternative method of providing controllably low molecular weight polymers that have desirably low ODs without the need for an additional synthetic step, for example, a method that does not result in the addition of absorptive terminal groups during the polymerization. In this manner, the need to compromise between OD and other molecular weight dependent properties, such as dissolution rate, is eliminated or at least reduced and the synthesis of the polymers is simplified. However, it should be realized that any such alternate method should also provide polymerizations having high conversion rates (greater than about 50%) while not substantially deprotecting acid labile groups (for positive tone resists) or deleteriously reacting with pendant groups such as hydroxyethyl ether (for negative tone resists) that are included as portions of the monomers selected for such polymerizations.